1. Field of the Invention
The invention relates to cardiac pacers. It has specific relevance to pacers capable of operating in dual chamber mode with ventricular stimulation and sensing and at least one of atrial stimulation and atrial sensing.
2. Description of the Prior Art
Stimulation pulses emitted by cardiac pacers must be of an energy to stimulate a contraction in the area of stimulation of a heart. If the energy of a pulse lies below the threshold level of the heart, no contraction will occur. In the past, clinicians were recommended to set the amplitude of the pulse to double the threshold amplitude value to ensure capture of the stimulation pulse. Recently, however, systems have been developed to allow this amplitude to be reduced to a level closer to the threshold value which results in obvious power savings. In such a system, the stimulation pulse energy can be reviewed, for instance periodically or on determination of a failed stimulation, to determine the optimal power level for reliable stimulation. This may be accomplished by progressively reducing the stimulating pulse energy over a number of stimulation cycles until no evoked response is detected. Once this critical energy level has been found, the stimulation pulse energy can be raised to the lowest level which resulted in an evoked response, with a safety margin of a few tenths of a volt added thereto, and this level is maintained until the next threshold check. Outside of this checking procedure, any stimulation pulse that fails to generate an evoked response will cause the pacer to raise the stimulation pulse energy until capture is attained once again. In both the procedures described above, a failure to evoke a response results in a backup pulse being emitted immediately to prevent discomfort to the patient. The backup pulse is a pulse of an energy level designed to guarantee a response in the heart.
In dual chamber cardiac pacers of the type described above, which include pacers operating in the DDI, DVI, DDD and VDD modes, it may be of interest to lengthen the interval between either the sensed atrial event or atrial stimulation pulse and ventricular stimulation to detect the underlying intrinsic rhythm of the heart. These intervals are commonly collectively termed the A-V interval. Correctly, the A-V interval refers only to the interval between atrial stimulation and ventricular stimulation, i.e. generation of an “A” pulse and a “V” pulse, while the interval between a sensed natural atrial event (a P-wave) and ventricular stimulation is called the P-V interval. However for the purposes herein, the collective expression A-V interval will be used to denote both situations unless otherwise specified. If the intrinsic rhythm of ventricular depolarization is acceptable, it may be beneficial to utilize this spontaneous activity rather than pacing the heart artificially in some situations, since this saves power. Thus if spontaneous ventricular activity is detected in the lengthened A-V interval, ventricular stimulation could be inhibited so that the spontaneous activity drives the heart while this is possible.
In the systems described above, however, wherein the pacer is configured to adapt its pacing pulse energy to the threshold of the heart, a ventricular stimulation pulse delayed to detect intrinsic activity may fail to evoke a response in the heart. This may be due to a momentary incorrect pulse energy level, or can occur, for example, when a spontaneous ventricular beat is coincident or almost coincident with the stimulation pulse and cannot be detected by the pacer. Such an event is commonly called a fusion or pseudo fusion beat. In order to ensure capture, the pacer will then transmit a backup pulse in the normal manner. Since the backup pulse is generated only after the pacer has determined that no response is evoked, there is an additional delay before the heart receives a successful pacing signal. This further delay might cause the pacer to detect a retrograde P-wave, possibly causing the heart to go into to pacemaker mediated tachycardia.
There is thus a need for a cardiac pacer operating in dual-chamber mode that is capable of operating in a power-saving manner while providing the heart with reliable and safe stimulation.